(For the background, my major, the first time I went to college was in book illustration, and the second time around it was graphic design renamed "visual communication".)

There's really no mystery here... and children have very little to do with it. In the world of book design, books for children are the kind that allows for the most creative approach, less constrained by expectations of the adult / professional world, and it's not going to turn into many-years project. So, in terms of how someone studies book design, it usually goes like this:

* the first year project is a book for children (I went with some folk fairy-tales, was pretty boring in the end)

* second year project it's a technical illustration book (I made a book about ballroom dancing, with the hallmark being feet-positions diagrams, was pretty lame)

* third year--something with difficult layout, like a play or encyclopedia (obviously, you don't make the whole book, just a few spreads) (for the life of me I cannot remember the project, I think it had to do with gothic dresses... maybe a history book about it)

* fourth year is something artistic (it's typical for this year for students to take on various kinds of printing, so the books would usually be made with woodcut / screen-printing / lithography / mezzo- or aquatint etc.), mostly for the benefit of showcasing the prints and less so to do any actual book design. (I made a series of woodcut illustrations for Moscow-Petushki by Venedikt Yerofeyev, was, actually, pretty decent, unlike the other three).

* And then the last year students choose whatever they want for their final project (I didn't graduate, but this would be often the case when students were already working in the industry, so they'd just bring the projects they worked on in their day jobs. Alternatively, some would choose to go down a more artistic line, and so won't make actual books, but something that had something to do with books, like again, a set of prints, or even just paintings. One guy, who in his final year transferred to art academy (from the printing academy) hand-made a Bible with a very interesting cover that was made in the shape of two hands pressed together as if for a prayer, for some reason that project was shown to the students year after year since he left.)

So, in conclusion: a children's book is the least demanding (in terms of technical knowledge about how to make books), and has the most opportunity to draw / paint / print. It's kind of like choosing acrylic over watercolors or oil paint: it's just easier to work with, fewer rules to follow, fewer expectations. So, for an artist, who isn't particularly skilled as a book designer, it's a natural choice (no artist would naturally choose a phone index as their first book!)

* * *

Oh, and to understand why books of all things... well, you need to first consider how different visual artists see each other. The only "true" artists are the painters, if you choose graphics, then you'd be labeled "colorblind", if you choose sculpture, you'd be labeled "blind", and history of art students are those who don't know how to draw and cannot learn. In this context, designers are the artists willing to compromise their artistic integrity for the crumbs falling off the rich people's table. Usually, this is because design work is done on a contract, with more or less assured payment, whereas most other artistic activities have very uncertain monetary reward (and usually none whatsoever).

Children books are good contracts in terms of time investment to pay ratio. They also don't require a lot of (if any) skill in making the product which is used by the art side to piggyback on (compare to, for example, designing furniture or buildings).